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What are Cells? - Overview

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  • 0:01 Cell Theory
  • 0:25 Cell Structure
  • 1:44 Categories of Cells
  • 2:55 Lesson Summary
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Lesson Transcript
Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

All living organisms are made up of cells, but do you know what a cell is? In this lesson, you can learn about the three main structures that all cells contain and about the two major types of cells.

Cell Theory

A cell is often called the basic unit of life. To explain the main characteristics of cells, scientists have developed the cell theory. Cell theory states three things:

  1. All organisms are made up of at least one cell.
  2. A cell is the smallest unit that can carry out all the functions necessary for life.
  3. All cells arise from pre-existing cells.

Cell Structure

While there are many different kinds of cells, all cells have three main parts:

  1. Plasma membrane
  2. DNA
  3. Cytoplasm

The plasma membrane or cell membrane is the outer portion of a cell. It's important for separating and protecting internal cellular structures from the surrounding environment. This allows a cell to have internal conditions that are very different from its surroundings.

This membrane is also important for allowing only certain substances to enter or leave the cell. This is called selective permeability. For example, nutrients have to be able to get into a cell so that it can survive. Waste substances must also be able to leave the cell.

In some cells, the plasma membrane is not the very outermost cellular structure. It is the cell wall instead. Cell walls are external to the plasma membrane and provide additional support and protection. They can be found in plants, fungi, and bacteria.

DNA is short for deoxyribonucleic acid. It is the molecule that makes up genes, which carry important instructions for keeping a cell alive. A gene is the unit of heredity which is transferred from parents to offspring.

Everything else inside a cell is called the cytoplasm. It's made up of a jelly-like water-based fluid called cytosol. There are other non-fluid substances found in the cytoplasm, but they vary between types of cells.

Categories of Cells

There are two main categories of cells: prokaryotic cells and eukaryotic cells.

Bacteria are very ancient and simple organisms. One individual is composed of only a single prokaryotic cell. These cells are very small and do not have any structures separating their DNA from the cytosol. In fact, they do not have any membrane-bound structures other than the plasma membrane. Additionally, the DNA of prokaryotic cells is arranged in a circle.

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